Scales

It is difficult to draw the components to their actual size, because they may be too large to be accommodated on the drawing sheet or too small to draw and cannot be effectively used in the shop floor. For example, think of making the drawing of motor car. It is too long and wide to be drawn on the drawing sheet to its original size. Similarly small component like a wheel of a wrist watch or its needle, if it is drawn to its original size will not be legible enough for use in the shop floor.

So depending upon the situation, drawings are drawn smaller or larger than the actual sizes. When we say that the drawings are smaller or larger, we mean that the given length in the drawing will be smaller or larger than the corresponding length in the object.

The ratio of the length in the drawing to its corresponding length of an object, when both the lengths are in the same unit, it is called the Representative fraction

R F =size of component in drawing /actual size of component c

Depending on the situation the term scale implies either RF or a measuring device itself made for a particular RF.

RF has two elements of which one of the elements is always ‘1’.

Example of RF: 1 : 5, 1 : 10, 150 : 1 etc.

First element in the RF always represents the size in the drawing while the second element represents the corresponding size of the object.